. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. DYNAMIC METEOROLOGY. 395 that is to say or for the limitiug case vy xUsT v=x RsT (2) (3) Each isotherm therefore commences ouly at a certain limiting point, whose abscissa is given by the equation (3) and whose ordinate is deter- mined by the relation found by combining equations (1) and (3) namely, 2) = e Ra + a; Rs . xUs (4) If the quantity x preserves a given constant value tUe isotherm con- tinues to lie iu its own plane as the tem
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. DYNAMIC METEOROLOGY. 395 that is to say or for the limitiug case vy xUsT v=x RsT (2) (3) Each isotherm therefore commences ouly at a certain limiting point, whose abscissa is given by the equation (3) and whose ordinate is deter- mined by the relation found by combining equations (1) and (3) namely, 2) = e Ra + a; Rs . xUs (4) If the quantity x preserves a given constant value tUe isotherm con- tinues to lie iu its own plane as the temperature T varies; the limiting point of tlie isotherm, as just defined, is displaced at the same time, and describes a curve that Bezold calls the line of saturation or the line of the deiv-point. (See Fig. 4.) This curve has its concavity turned toward the side of positive i?. The indicator point for air in the dry stage ought therefore always to be on the concave side of the line of saturation ; if this point passes over to the convex side it indicates that the dry stage has been followed by the rain stage. When the quantity x varies while T remains constant the projections upon the plane of co-ordinates of the isotherms corresponding to the various values of x sensibly agree with each other, as we have said, at least when one draws a diagram rather than a rigorously exact figure. On the other hand, the limit- ing point in this common i)rojection on the plane of oo-ordinates is not the same when we take different values of x. We find, without diffi- culty, that if Vi and ^2 are the abscisses of the limiting points belong- ing to the quantities of vapor Xi and X2, respectively, we have ^=^» that is to say, that the abscissas of the limiting point vary propor- tionately to X. To each value of x there corresiwnds a line of satura- tion, precisely as to each value of T there corresponds anisothern. Adiabatics.—The equation of an adiabatic in the dry stage is. Fig. 4.—Dew-po
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